Sean K. Ellis spent nearly 22 years in prison for the 1993 murder of Boston Det. John Mulligan. Ellis claimed innocence, and in 2015 the courts overturned his conviction, ruling "justice was not done" (unanimously upheld by the Mass. SJC in 2016). A retrial -his 4th- is scheduled for Oct. 18, 2018.

CO-DEFENDANT FREED

Key evidence disallowed, Patterson is released

Murder victim John Mulligan was a controversial cop, the subject of more than a score of police misconduct probes and multiple lawsuits for citizen brutality and false arrest. His heavy-handed policing style and his gangland-style slaying led police to presume a revenge motive, perhaps by someone he'd wronged. Yet Ellis's teenage co-defendant, Terry Patterson, was speedily convicted of his murder and robbery in February 1995 , after fingerprints that police said were his were found on the door frame of Mulligan's vehicle.

In 2000 the SJC reversed his murder and robbery convictions based on a conflict of interest by his trial attorney that, ironically, concerned police perjury (see "Trials"). He was moved from maximum security Walpole State Prison to Nashua Street Jail to await a retrial.

In 2004, Boston defense attorney John H. Cunha, Jr. motioned Suffolk Superior Court to discount the "latent" fingerprint evidence found on Detective Mulligan's car that was used against Patterson. No single latent impression, on its own, could reliably be matched to any corresponding finger on Patterson's hand. Attorney Cunha argued that the Boston Police method of collecting "simultaneous latent prints" from four fingers and adding up their "ridge characteristics" to create a match was invalid, for there were not enough points of comparison for each fingertip.

In December 2005, the Massachusetts Supreme Judicial court agreed with attorney Cunha that the practice used by Boston Police was constitutionally unreliable and not based on generally accepted scientific theory.

The "latent prints" purported to be Patterson's were disallowed as evidence -- a stunning legal victory with wide implications, for the court ruled that no prints collected by this method could henceforward be admitted as evidence in the Commonwealth of Massachusetts.

With the fingerprints the Commonwealth claimed were Patterson's now in the trash bin, the case against him fell apart. Prosecutors crafted an agreement that would allow Patterson to plead guilty to manslaughter, armed robbery, and gun charges, and in return be freed from prison, credited for time served.

Like Sean, Patterson had always insisted he was innocent of Mulligan's murder. Now he was looking at an open prison door if he would sign the Commonwealth's affidavit admitting culpability.

He signed.

Terry Patterson. In May 2006 he was freed from prison after pleading guilty to manslaughter and being credited for time served.

Free at last, June 3, 2015. Sean Ellis leaves Boston's Suffolk Superior Court holding his mother, Mary's, hand. Arrested at age 19, he spent more than half his life in prison for a crime he maintains he did not commit. (Getty Images)